• Closing capacity is the smallest lung volume that is required to prevent closure of small airways or alveoli. Closing Capacity (CC) = Residual Volume (RV) + Closing Volume (CV)
• The ability to recognize patients at risk for decreased functional residual capacity (FRC) or increased CC can be useful to predict the degree to which intraoperative management will affect airway closure, shunt, Alveolar-arterial (A-a) gradient.

• Bronchioles distal to the 11th generation are 1 mm do not have rigid cartilaginous support to keep them open. These distal airways are subject to opposing forces that can prevent or maintain patency (1):
  • Outward elastic recoil of the chest wall (preventing airway collapse) the opposing inward pull of the lung parenchyma; the sum of these opposing forces determine the intrapleural pressure (Ppl) at varying lung volumes. Note: The FRC is equal to the volume at which the Ppl is zero.
  • Surfactant maintains small airway patency by decreasing surface tension collapse.
  • If the CC is greater than the total lung volume at any given point (inhalation or exhalation), premature airway closure results in distal lung segments that continue to receive perfusion, but are no longer ventilated. The shunting of unoxygenated blood from these segments into the arterial circulation can contribute to hypoxemia.
• CC describes the lung volume at which any further exhalation results in airway closure. CV is the volume of air that can still be exhaled from the point of CC until only RV remains, during maximal exhalation. CV is usually less than the functional residual capacity ( expiratory reserve volume) in a healthy young adult in the sting position.
• FRC is the resting point at the end of normal exhalation where elastic recoil of the chest equals inward lung recoil. It is the volume of air that remains in the lungs at the end of a normal tidal volume (TV) exhalation; FRC = ERV + RV.
  • RV is the volume of air left in the lung after a maximal expiration. During forced expiration, before exhalation to RV, small airway closure begins to occur as the CC is reached.
• CC versus FRC.

  • CC FRC. Small airways remain patent during normal TV. In young healthy patients, lung volume at the end of resting exhalation (FRC) is greater than CC (Figure 1).

    

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    FIGURE 1. Normal pulmonary function its relationship to closing capacity.

    CC is represented by the dotted line.

  • CC > FRC. If pathological or mechanical changes result in a reduction in FRC or an increase in CC, airway closure may occur during normal TV breathing (Figure 2).

    

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    FIGURE 2. A graphical representation of respiratory function in a person with decreased FRC /or increased CC.

    Note how normal TV breaths result in lung volumes below CC. CC is represented by the dotted line.

• Since CC = CV + RV, changes in RV affect FRC CC equally (FRC = ERV + RV). Thus, either a decrease in ERV or an increase in CV closes the gap between FRC CC. An increase in CC signifies that airway closure occurs at higher lung volumes; in other words, while exhaling toward RV, closure occurs earlier.
  • CV increases with age, smoking, COPD (2); factors that affect lung parenchyma elasticity. These airways become dependent on greater lung volumes to keep them open; as a result the CC rises (1). Additionally, it should be noted that CV is not dynamic like ERV FRC is unaffected by body positioning.
  • ERV ( FRC) is decreased in obesity, head down positioning, general anesthesia as a result of decreased lung compliance. This is caused by pressure from adipose tissue on the lungs, diaphragm, abdomen (3).

• Bronchioles
• Alveoli
• Ribs
• Muscles of respiration
  • Diaphragm
  • Intercostal muscles
  • Scalene muscles
  • Sternocleidomastoid muscles

• Age, smoking, lung disease can lead to a decrease in lung recoil (the force pulling the lung into itself) an increase in lung compliance.
• Changes in collagen fibrils elastin in the lungs account for the increase in compliance permit the lungs to inflate to larger volumes with less pressure. This results in larger lung volume requirements to keep small airways open increases CC (1). Early airway closure, like atelectasis, can result in ventilation/perfusion (V/Q) mismatch where areas of unventilated lung may continue to receive perfusion. Areas of perfusion without ventilation, also known as shunt, can contribute to hypoxemia.
• Aging increases both RV CV. While changes in RV affect CC FRC equally, an increase in CV will increase CC toward FRC (Figure 3).
• CC, while supine, increases to equal FRC in one's mid-forties becomes greater than FRC, even when sting, in one's mid-sixties. Eventually, exhalation of normal TV can bring lung volumes below CC resulting in airway closure during normal breathing (4).
• Obesity does not affect the CC. It, however, diminishes chest lung compliance, which ultimately decreases FRC. This reduction in FRC results from a reduction in ERV; RV is not affected (3). An obesity-associated reduction in FRC can result in lung volumes below the CC early airway closure.

• Decreased FRC from supine positioning general anesthesia can result in early airway closure when FRC falls below CC (CC > FRC).
• General anesthesia reduces FRC by 0.4–0.5 L (5) [A]; this is presumed to occur from a loss of respiratory muscle tone decreased chest wall compliance. Additionally, laying supine can reduce FRC by up to 1 L. When the FRC is reduced in this manner below CC, air trapping shunting occurs. This has been hypothesized to be the cause of increased A-a gradient [P(A-a)O₂] during surgery (6). This effect is further enhanced in patients with a baseline diminished FRC or elevated CC. Position anesthesia do not affect the CC; however, they reduce the FRC, bringing it closer to the CC.
• PEEP is often implemented to increase the FRC improve the A-a gradient [P(A-a)O₂] (5) [A]. However, its use is limited since PEEP affects upper lung zones greater than lower zones. Conversely, airway closure occurs to a greater degree in the lower lung zones. In addition, an increase in intrathoracic pressure from PEEP may impede venous return result in deterioration of cardiac output.
• “Alveolar recruitment” may be achieved by applying a limited number of double TV breaths in an attempt to reopen closed airways alveoli. An inflation pressure up to 30–40 cm H₂O may be required (7) [A]. This must be done with caution as hemodynamic instability can result from an increase in intrathoracic pressure decreased venous return.
• Sitting a patient upright, or reverse Trendelenburg, >30° may increase FRC. This is useful when trying to maximize respiratory function in the immediate postoperative period including extubation.
• FRC remains decreased into the postoperative period. With reduced FRC, lung volumes are more likely to fall below CC during resting TVs, contributing to post-operative hypoxemia.

• CC = CV + RV
• FRC = ERV + RV

Kenneth R. Moran , MD